1. Field of the Invention
The present invention relates to radio frequency (RF) repeaters. More particularly, the present invention relates to stability recovery system and method for an on-frequency RF repeater.
2. Description of the Prior Art and Related Information
In a wireless communication system, a mobile unit such as a cellular phone transmits and receives radio frequency (RF) signals to and from cell site base stations. An on-frequency RF repeater receives, filters, and re-transmits the signal of interest at the same frequency and at a higher power level, thus extending base station coverage. If the product of the forward gain of the repeater and coupling between antennas is too high, the repeater will oscillate. In general, this is undesirable. An internal feedback path may be used to compensate for the external coupling between antennas, allowing the forward gain of the repeater to be increased if required. This compensation is referred to as “echo cancellation.” However, if the forward gain is increased too high compared to the residual feedback coupling after echo cancellation, the repeater becomes unstable.
Some residual echo after cancellation is acceptable. However, it is desirable to minimize such residual echo. This may be difficult in some applications, especially for large repeater gains or other repeater implementations having difficult cancellation conditions. In the past, repeaters without echo cancellation need to be monitored for stability, and if instability was detected, the forward gain was reduced to return stability. However, for a repeater with digital echo cancellation, both the analog gain and the coefficients of the echo cancellation must be adjusted to restore stability. Therefore, a method is required to restore stability taking into account both the analog gain and the echo cancellation coefficients without requiring the echo cancellation to be reset to zero.